rustpkg: Fail when crate inference fails; inject link attributes

1. Fail when there's no package script and no crates named
main.rs, lib.rs, bench.rs, or test.rs.

2. Inject the crate link_meta "name" and "vers" attributes, so
that the output file gets named correctly in the library case.

3. Normalize '-' to '_' in package names.
rustpkg: Add a few tests

There's no test runner for rustpkg yet; just sketching out a few
basic test scenarios. pass/ contains packages that should compile
successfully, whereas fail/ contains packages that should fail
to build.